This invention relates to an apparatus and method utilizing a beta gauge for regulating the production of a continuous web of material such as paper, and more particularly to the augmentation of such a system with a radiation inspection system which continuously monitors the entire web and compares the signal generated thereby with the signal generated by the beta ray gauge in order to control the formation of the web of material being produced.
Generally, the paper making process involves spreading the stock formed of a slurry of water suspended fibers on a wire mesh in the form of a cylinder or table. The wire mesh forms a support and provides drainage through it. The stock is applied to the wire mesh by a flowspreader which converts a flowing stream of stock into a flat thin stream having a uniform velocity across the width of the machine. The web so formed on the wire mesh is passed through a press section which removes more of the water and then applied to a dryer section where the remainder of the moisture is removed.
The paper making process is implemented with the foregoing as well as other apparatus being incorporated in huge paper making machines which operate continuously and turn out tons of paper products. Although the paper making process performed by the machines has had the production process computerized, the paper making art is still empirical by nature. Accordingly, any improved methods of controlling the uniformity and product quality of the paper produced by the paper mill would be highly desirable.
One of the measures which is used in the quality control process is the basis weight of the resulting paper product. One way of determining the basis weight is to use a beta ray gauge which includes a measuring head which is suspended on a monorail and so positioned that the continuous web of paper is passed through a gap in the beta ray gauge. The beta ray gauge includes a beta ray source which passes radiation through the web to a detector which includes an ionization chamber for detecting the beta rays passed by the paper web. As the weight of the paper product increases, fewer beta rays reach the detector and as the weight decreases, more beta rays are detected. The detected signals are automatically compared with a reference signal which has been preset for the weight desired. The difference between the two signals is measured and deviations above and below the desired weight indicate a heavy or light sheet. Automatic correction then can be applied by coupling a controller which makes the comparison to a stock valve, gate positioners, or consistent regulators of the flowspreader for controlling the dispensing of the paper stock on the wire mesh.
However, one of the problems of this approach is that the beta ray gauge measures only a sample area on the continuous web, and accordingly any comparison made would only be valid for that particular sample. Some of the paper processes accordingly provide a scanning beta ray gauge. However, the problem with this approach is that the gauge moves slowly and does not cover the entire width of the continuous web and even if the gauge did scan the entire area of the web, the cloud chamber response time of the gauge detector is so slow that changes in basis weight across the web as well as down web cannot be measured. Additionally, the minimum spot size obtainable by a practical beta ray gauge is relatively large, e.g. on the order of 0.125" while spot sizes of 0.010" and smaller are practical with an optical scanner allowing for measurement of formation as well as basis weight. Formation is the fiberous structure and relates directly to how well paper can be printed, coated, its tear strength and its visual appearance.